To better understand the severity of a particular condition at an anatomical structure of a patient, physiological data can be gathered and used to guide treatment decisions. One example of such a condition is a constriction, or narrowing, of a blood vessel, referred to, in some cases, as a stenosis. By gauging the severity of the constriction, appropriate treatment options can be determined.
One technique for evaluating the degree to which a stenosis obstructs flow through a blood vessel is called the Fractional Flow Reserve measurement (FFR). To calculate FFR for a given vessel, two blood pressure readings are taken—one on the distal side of the stenosis (e.g., downstream from the stenosis) and the other on the proximal, or aortic, side of the stenosis (e.g., upstream from the stenosis, toward the aorta). FFR is defined as the ratio of maximal blood flow in a stenotic artery, taken distal to the stenosis, to normal maximal flow, and is typically calculated based on a measured pressure gradient of the distal pressure to the proximal pressure. The pressure gradient across a stenosis may serve as an indicator of the severity of the stenosis. The more restrictive the stenosis is, the greater the pressure drop, and the lower the resulting FFR. FFR measurement may be a useful diagnostic tool. A physician might decide, for example, to perform an interventional procedure (e.g., angioplasty or stent placement) when FFR for a given stenosis is below a clinical threshold (e.g., 0.8), and may decide to forego such treatment for a given stenosis where FFR is above the clinical threshold (e.g., 0.8). Thus, FFR measurement can be a decision point for guiding treatment.
However, accurate assessment of the pressure drop at a stenosis generally requires that coronary resistance be stable and minimized. In traditional FFR, this has generally been achieved by inducing maximal hyperemia in the vessel through administration of a pharmacological hyperemic agent, such as adenosine. It would be preferable to make an approximation of FFR under normal flow conditions without needing to administer a pharmacological agent since this could reduce patient side effects as well as cost and time associated with the diagnostic assessment.
Techniques have recently been developed to make an approximation of FFR at a time when coronary resistance is naturally minimized and thus a pharmacological agent is not needed. These techniques have focused on identifying the diastole period of the cardiac cycle and taking pressure measurements during a defined sub-period of diastole when resistance has been empirically shown to be low. But, in order to identify the diastole period, these techniques rely on first identifying the dicrotic notch in the pressure measurements. The dicrotic notch represents closure of the aortic valve at the onset of ventricular diastole and appears in the pressure waveform as a relatively slight, upward deflection in a descending portion of the pressure waveform. However, depending on the particular patient, the dicrotic notch can be difficult to detect and, in some cases, there may be no discernable dicrotic notch at all. Since these FFR approximation techniques define the pressure measurement period relative to the dicrotic notch, failure to accurately identify the dicrotic notch can lead to use of pressure measurements taken when vessel resistance is material and thus result in an inaccurate approximation of FFR. This, in turn, may reduce the value of the FFR approximation in guiding treatment.